The present invention relates to calibrating a nondispersive infrared photometer, and more particularly the invention relates to a calibrating device for use in conjunction with a nondispersive infrared photometer; such a photometer includes an infrared radiation source and a radiation modulator; and a twin or double sample chamber with symmetrically configured measuring and reference ray paths for purposes of selective absorption of infrared radiation passing through. The photometer further includes a receiver and a filter device for the infrared radiation to provide for certain selectivity in the detection process. This kind of a photometer is to be calibrated by use of calibration chambers which contain particular gases to be placed into the radiation path mentioned above, whereby specifically for purposes of measuring different chambers each being filled with an inert gas are placed in the respective optical path, while during calibration operation an alternate chamber pair is shifted into the radiation path wherein in one chamber a calibration gas is provided while the other one, a reference chamber, is provided with an inert gas.
Within the NDIR photometers of the type to which the invention refers and pertains broadly, a radiation source is used which is basically a thermal radiator. The receiver on the other hand is usually a gas filled absorption chamber or a solid state detector. The absorption path is provided, in the case of extractively operating photometers, in accordance with the measuring principles outlined above. The absorption path is constructed to include twin sample chamber as stated. NDIR photometers of the type to which the invention pertains are by their very nature subject to certain temporal drift, aging etc. The need exists to recalibrate them from time to time.
Recalibrating was carried out in the past in accordance with one kind of state of the art teaching, by using the measuring sample chamber and reference sample chamber of the equipment in the normal course of events but charging it with test gases of known properties. These test gases of course have to be certain and predetermined as to their properties and are usually maintained in metal bottles, flasks or the like which are usually quite heavy. This simple fact renders calibration, particularly recalibration in the field, quite expensive and cumbersome.
German patent application No. 35 22 949 suggests a calibrating chamber which includes a test gas that is sealed therein and from time to time this replacement calibrating chamber is just put into the equipment for test purposes. Peculiarly enough this kind of equipment has not found acceptance, at least it is not generally accepted in NDIR meters. It is believed that the reason for this lack in success results from the fact that the photometers use thermal radiation sources and are thus very sensitive against thermal and optical changes based on thermal changes and vice versa. This is particularly true in the case of a two beam photometer of the type referred to above having a measuring beam path and a reference beam path and wherein a differential signal is produced from signals attributed respectively to the detected measuring beam and to the detected reference beam. Putting a calibrating sample chamber system into the radiation path was found to interfere with the equilibrium of the photometer as a whole owing to the different optical and thermal effect measuring and reference beams undergo. Hence, the resulting measuring signal is in fact distorted during the calibration procedure which of course defeats its purpose. In addition radiation is attenuated owing to a different number of optically effective surfaces of the calibration equipment on one hand and the regular equipment on the other hand.
German patent No. 28 26 522 describes a calibration device wherein a single calibration chamber is shifted into the measuring path or pivoted into the path, while the reference path remains as is. Hence, the problem of optical and thermal symmetry and coupling are not subject to the calibration procedure nor are losses through the gaps or the like taken into consideration. These gaps do produce certain losses and thereby reduce the sensitivity of the equipment. Penetration of air containing CO.sub.2, for example from the environment, will definitely interfere with CO.sub.2 measurement through uncontrolled preabsorption. Other gases even of unknown consistency that may be happening in the air will compound the problem.